Lecture 13 - Co-factors and Coenzymes Flashcards
Give a breif history of Enzymology?
Enzymology is an old discipline
* Ancient application of biological catalysis - fermentation
* 1700 - early 1800s: early biological studies of catalysis
○ 1700s digestion of meat by stomach secretions
○ 1800s starch-to-sugar by saliva and plant extracts
* Victorian era
○ Origins of the term enzyme meaning “leavened”, “to modify or transform” [Emil Fischer]
○ Specificity - Lock and key hypothesis [Wilhelm Khune]
* 1920s: crystallisation of urease identifies protein as a source of enzyme activity [Hames Sumner]
* 1960: Advent of X-ray crystallography facilitates structure-based studies of enzyme activity and regulation.
* 1980: Catalytic RNA molecules discovered [Thomas Cech]
In 1930 J.B.S Haldane wrote the article “Enzymes”. In this text he suggested that multiple weak bonding interactions between an enzyme and its substrate(s) might be used to catalyse reactions.
What is the function and the differences of co-factors and co-enzymes?
Some enzymes require no chemical groups other than their amino acid residues
* Co-factors - Are non-protein components required for enzyme activity
* Co-enzymes - Organic cofactors
Cofactors can be tightly or loosely associated with their enzymes. Tightly associated cofactors are sometimes known as prosthetic groups
Co-enzymes that are loosely associated with their enzymes are often referred to as cosubstrate
Another common group of cofactors are metal cations. The multiple oxidation states of some metal atoms are often important for their cofactor function.
What is the function of co-enzymes in group transfer reactions?
They are transient carriers of functional group information
For enzymes with a tightly associated co-enzyme or co-factor (prosthetic group)
* In the absence of cofactor the protein is referred to as an apoprotein or an apoenzyme (Inactive)
In the presence of a bound cofactor the protein is known as the holoprotein or holoenzyme (active)
Name some cofactors and the vitamins they are derived from.
Vitamin B3 NAD+/ NADP
Vitamin B2 FMN/FAD
Vitamin B5 Coenzyme A
Vitamin B6 Pyridoxal phosphate
Describe the function of NAD+ and NADP in 2e- transfer reactions.
NAD+ and NADP both have a nicotinamide ring derived from niacin. NAD+ and HADP both undergo reversible reduction of their nicotinamide ring. Therefore, for a substrate undergoing enzyme-catalysed oxidation the oxidised form of the co-enzyme accepts a hydride ion (has two electrons)
NAD+ and NADP differ in their R group
* NAD+ = H
* NADP+ = PO32
NAD+ functions primarily in the reversible oxidation of aldehydes and alcohols
This is what allows us to drink alcohol as NAD+ with the alcohol dehydrogenase enzymes forms the toxic Acetaldehyde which is then converted to CH3CHOO- which is much less toxic by aldehyde dehydrogenase and NAD+
Reduction converts benzenoid ring of nicotinamide moiety (fixed +ve charge on N) to quinoid form (no charge)
Significance of the typical intracellular NAD/NADH and NADP/NADPH ratios
* The ratio of NAD/NADH is high as it favours hydride transfer from substrate
* NADPH/HADP high - favours hydride transfer to substrate
Stereochemistry of the two hydrogens on the reduced nicotinamide ring of NADH or HADPH are different. A dehydrogenase will always use the H on the same side of the nicotinamide ring however which will be used will depend on the enzyme.
The specificity is determined by interactions with amino acid side chains within the co-enzyme binding pocket.
What is the function of high potential electrons in biosyntetic pathways?
High potential e- are required in many biosynthetic pathways as precursors and pathway intermediates are generally more oxidised than products
E.g. Reductive Biosynthesis.
The most common electron donor in biosynthetic pathways is NADPH which is used predominantly for reductive biosynthesis. The P (phosphate) provides a tag of specificity
A major source of NADPH within cells is the pentose phosphate pathway
NAD+ functions primarily in oxidation of alcohols and aldehydes
As a general rule enzymes are very specific for either NAD or NADP there are exceptions such as trans deamination catalysed by glutamate dehydrogenase which will use either
How do enzymes lower the activation energy?
- Enzyme binding substrate results in a reduction in the degrees of freedom of movement/rotation
- Enzymes distort the substrate towards the transition state
- The sum of the multiple weak bonding energies upon interactions between individual residues at the active site and the substrate can offset/pay for the activation energy
What is the role of flavins in e- transfer reactions?
They can participate in either 1 electron or 2 electron processes.
FMN = flavin mononucleotide
FAD = Flavin adenine dinucleotide (FMN + AMP Unit)
Flavins are derived from riboflavin (vitamin B2). The reactive part of FAD/FMN is the isoalloxazine ring. They participates in desaturation reactions.
Flavins participate in either 1 electron or 2 electron processes
* Flavin groups do not transfer electrons by diffusion form one enzyme to another. Instead the flavin is a prosthetic group and provides a means by which a flavoprotein temporarily holds electron transfer from a reduced substrate to an electron acceptor.
* In complex I FMNH2 is reoxidised by two successive 1 e- transfers to Fe3+
What is co-enzyme A derived from and what is purpose?
Coenzyme A is derived from pantothenic acid (Vitamin B5)
CoA(SH) provides an activated carrier of acetyl and acyl groups
Describe the similarity betwen common coenzymes and ATP
There are similarities between the structures of common co-enzymes and ATP
* The Rossman fold is a common fold found in enzymes (dehydrogenases) used to bind co-factor - a classic nucleotide binding motif found in all NAD(P) dependent dehydrogenases and used to bind the cofactor.
